1. Field of the Invention
The present invention relates to dielectric filters using dielectric members having resonance lines formed thereon or therein, duplexers, and communication apparatuses incorporating the same.
2. Description of the Related Art
Conventionally, a dielectric filter including a plurality of resonance lines formed on a dielectric substrate or inside a dielectric block is used as a band pass filter in a communication apparatus such as a mobile phone.
Japanese Unexamined Patent Application Publication No. 11-340706 provides a dielectric filter in which the attenuation-pole frequency of the filter can be freely set and good preferred characteristics can be obtained with a simple structure.
In the dielectric filter, an attenuation pole is generated by connecting input/output terminals to positions deviated from the center of a resonator in the direction of one of its end faces, that is, by so-called tap coupling.
In the dielectric filter having input/output terminals with tap coupling, according to the positions of tap coupling with the resonators, the position of a generated attenuation pole can be set over a relatively wide range. Thus, there is an advantage in that preferred passing characteristics and attenuation characteristics can be more freely set. However, the structure of the resonator inherently dictates the positional relationships between the pass band and the attenuation pole, for example, whether an attenuation pole is generated on the high frequency side or the low frequency side or whether it is generated on both sides. As a result, there are limitations to the freedom to generate attenuation characteristics on the high frequency side and the low frequency side.
The present invention provides a dielectric filter, a duplexer, and a communication apparatus which avoid these limitations. The dielectric filter can obtain arbitrary passing characteristics and attenuation characteristics by generating many more attenuation poles, in addition to attenuation poles generated by tap couplings.
According to a first aspect of the present invention, there is provided a dielectric filter including a dielectric member, a ground electrode and a plurality of resonance lines formed on the dielectric member, and input/output units tap-coupled with the resonance lines. In this filter, predetermined resonance lines are disposed adjacent to each other to permit distributed constant resonator coupling so that a first attenuation pole is generated on one of the high frequency side and the low frequency side of a pass band, and the tap coupling permits a second attenuation pole to be generated on one of the high frequency side and the low frequency side of the pass band.
As mentioned here, attenuation characteristics obtained on the high frequency side and the low frequency side can be arbitrarily determined by setting either one or both of the first attenuation pole generated by the distributed constant resonator coupling and the second attenuation pole generated by the tap coupling onto the high frequency side, the low frequency side, or both sides of the pass band.
Furthermore, in addition to the second attenuation pole generated by the tap coupling mentioned above, the invention permits attenuation poles to be generated on the high frequency side and the low frequency side by capacitive coupling and inductive coupling between resonators. In this filter, one end of each resonance line may be an open-circuited end and the other end thereof may be a short-circuited end.
Additionally, the resonance line may have a stepped structure in which the line width of the open-circuited end is differentiated from the line width of the short-circuited end. In this case, since there is no need for a special electrode to couple the resonators, attenuation characteristics on the high and low frequency sides of the pass band can be freely determined.
In addition, in this filter, the first attenuation pole obtained by distributed constant resonator coupling may be generated on the low frequency side, and at least two second attenuation poles obtained by tap coupling may be generated on the high frequency side. With this arrangement, for example, a spurious mode response appearing on the high frequency side of the pass band can be suppressed.
In addition, in this filter, all of the first attenuation pole obtained by the distributed constant resonator coupling and the second attenuation poles obtained by the tap coupling may be generated in mutually adjacent positions on the high frequency side or the low frequency side. This arrangement can provide large attenuation at the two attenuation poles.
In addition, two attenuation poles of one type, such as second attenuation poles obtained by tap coupling, may be generated respectively on the high and low frequency sides, and an attenuation pole of another type, such as a first attenuation pole obtained by distributed constant resonator coupling, may be generated respectively on the low or the high frequency side.
Furthermore, in this filter, one end of each of the resonance lines may be an open-circuited end and the other end thereof may be a short-circuited ends, to form a xc2xc-wavelength resonator. Or, both ends of each of the resonance lines may be short-circuited ends to form a xc2xd-wavelength resonator. With this arrangement, at least two attenuation poles generated by tap coupling can be obtained on the high frequency side of the pass band.
Furthermore, in the dielectric filter of the invention, both ends of each resonance line may be open-circuited ends, to form a xc2xd-wavelength resonator. This arrangement permits attenuation poles to be generated on both of the high frequency side and the low frequency side.
Furthermore, the dielectric member may be a substantially rectangular parallelepiped dielectric block. Inside the dielectric block there may be formed through-holes having inner conductors disposed on the inner surfaces thereof to constitute the resonance lines. With this arrangement, since the Q0 of the resonator can be increased, unnecessary coupling between the resonance lines and the outside can be prevented.
In addition, in this filter, the input/output units may include input/output terminal electrodes disposed on outer surfaces of the dielectric block and conductive films disposed in lateral holes continuing from the input/output terminal electrodes to predetermined positions of the through-holes. With this arrangement, the lateral holes can be formed and the conductive films can be disposed on the inner surfaces of the lateral holes in the same manner as the formation of the through-holes and the addition of the inner conductors on the inner surfaces of the through-holes. This arrangement facilitates tap coupling.
According to a second aspect of the present invention, there is provided a duplexer including two dielectric filters as described above, for use as a reception filter and a transmission filter, and input/output terminals for being connected to a common antenna, which are disposed between the two dielectric filters.
In addition, according to a third aspect of the invention, there is provided a communication apparatus including the dielectric filter or the duplexer as described above, which is used for selectively passing/blocking signals.